[Rhodium-diolefin-phosphorus ligand] complexes find use in catalysis, for example in hydrogenation reactions, where the use increasingly requires low residual levels of impurities. In known methods, [rhodium (diolefin) halide]2 starting materials have been frequently used where the halide anion has been exchanged using Ag, Tl, alkali or ammonium salts of the required anion. On applying any of these methods, we have found a variety of contaminants in the product.
In one approach, [rhodium (diolefin) halide]2 is treated in the presence of further diolefin with Ag or Ti salts of the required anion forming [rhodium (diolefin)2]+ compounds. These rhodium compounds retain intolerable quantities of Ag or Tl residues which have a detrimental effect on storage stability. The impurities are also present when the [rhodium (diolefin)2]+ compounds are reacted with the phosphorus ligands to form [rhodium-diolefin-phosphorus ligand] complexes. In addition to the above problems, low yields of the complex are often obtained and whenever Ag or Tl salts are used, recovery of rhodium from the hydrogenation liquors and refinery is particularly costly because of the need to separate rhodium from the other metals during refining.
In another approach, [Rh(diolefin) halide]2 is treated with the phosphorus ligand followed by alkali or ammonium salts of the required anion. The use of alkali and ammonium salts normally results in residual amounts of halide which can also limit storage stability, as well as having a detrimental effect in the catalytic application of the rhodium complex. Furthermore, the obtained [rhodium-diolefin-phosphorus ligand] complexes often contain unacceptable amounts of cationic tetracoordinate rhodium contaminants where the diolefin has been replaced by additional phosphorus ligand(s).
Another method of preparation converts [rhodium (diolefin) halide]2 to halide free rhodium-diolefin-1,3-diketonate complexes using known, methods. The rhodium-diolefin-1,3-diketonate complexes, such as Rh(diolefin)(acac), are mixed with a phosphorus ligand and a strong acid (HClO4) in tetrahydrofuran solvent to obtain rhodium-diolefin-phosphorus ligand complexes (see R. Schrock, J Osborne, J. Am. Chem. Soc. 1971, 93, 2397-2407). One problem with the use of strongly coordinating ethereal solvents such as tetrahydrofuran (THF) has been observed by M. D. Fryzuk and B. Bosnich who used [Rh nbd acac] in THF for the synthesis of [Rh nbd (S,S)-Chiraphos]ClO4 THF adduct (see J. Am. Chem. Soc. 1977, 99, 6262-6267). A THE adduct of the complex was obtained and it proved impossible to remove the THF from the obtained complex.
WO 2005/032712 discloses a method for preparing rhodium phosphine complexes comprising the steps of (a) dissolving Rh(diolefin)(acac) in a strongly coordinating ethereal solvents such as tetrahydrofuran (THF) and additional ethereal solvents such as diethyl ether or methyl tert-butyl ether (MTBE) (b) adding to this a fluorinated non-mineral acid HX, such as a tetrafluoroboric acid etherate, and alcohol solvent or alcohol containing solvent mixture, either simultaneously or sequentially, to form a soluble solvated complex of rhodium with one or more of the reaction solvents, (c) adding the phosphorus ligand, either in solution in an organic solvent or neat, and (d) collecting the crystalline precipitate. This procedure is not satisfactory because two ether solvents are required in addition to the alcoholic solvent and a number of the examples describe crystallisation processes at very low temperatures such as −20 to −30° C.
EP1127061B1 discloses a procedure for the preparation of slurries of [Rh(COD)2]BF4 from a solution of Rh(COD)acac in THF. The procedure is difficult to scale up because the slurry of [Rh(COD)2]BF4 intermediate precipitates from THF and it is extremely difficult to agitate on a large scale. After the addition of the phosphorus ligand, the soluble cationic [rhodium diolefin phosphorus ligand] complex is precipitated by adding ethereal anti-solvents like MTBE and diethyl ether.